1. Field of the Invention
The present invention relates to a flow meter, and more particularly to a three pickoff sensor flow meter.
2. Statement of the Problem
Vibrating conduit sensors, such as Coriolis mass flow meters and vibrating densitometers, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flow meter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes including, for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Two such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. The two pickoff sensors are connected to electronic instrumentation by cabling, such as by two independent pairs of wires. The instrumentation receives signals from the two pickoff sensors and processes the signals in order to derive a mass flow rate measurement.
Flow meters are used to perform mass flow rate measurements for a wide variety of flow fluids. One area in which Coriolis flow meters can potentially be used is in the metering and dispensing of fuels, including alternative fuels. The alternative fuels market continues to expand in response to increasing concerns over pollution and further in response to increasing concerns about the cost and availability of unleaded gasoline and other traditional fuels. In fact, many governments are becoming involved by enacting legislation promoting the use of alternative fuels.
An opportunity for the use of Coriolis meters in the alternative fuels market is in the filling of vehicles, such as cars, buses, etc. In the prior art, the filling of individual vehicles has been performed at filling stations utilizing traditional gasoline pumps or utilizing compressed natural gas (CNG) dispensers for alternative fuels. Traditional gasoline fuel dispensers require two individual and independent meters so that two vehicles can be filled simultaneously. A dual meter fuel dispenser can provide two metered flow streams. The two flow streams can flow at different rates. The two flow streams can be of different flow materials (i.e., two different fuels, for example) and can have different densities.
However, the overall cost and size of a fuel pump for an alternative fuels must be minimized in order for the manufacture of the pump to be competitive in such a growing industry. Therefore, a challenge exists in being able to develop a cost-effective fuel meter that can provide two simultaneous fuel flow measurements on two independent flow streams.
One prior art approach is to install two separate flow meters in such a fuel dispenser. Although this is a workable approach, it has drawbacks. Two meter devices take up double the space in the fuel dispenser as a single meter device. Two meter devices can double the meter expense of a fuel dispenser. Two meter devices can require double the electrical power. Two meter devices can require double the number of dispenser components, such as solenoid valves, regulators, check valves, piping, etc.